allenap-flock-experiment

Simple interactive tool to experiment and learn about flock which is available on Linux, macOS, and other Unix-like systems. It's behaviour can be somewhat subtle, and I found it helpful to get hands-on experience with it when writing about flock in 2017.

Command-line utility

If you have installed Cargo, you can install allenap-flock-experiment with cargo install postgresfixture. This puts a allenap-flock-experiment binary in ~/.cargo/bin, which the Cargo installation process will probably have added to your PATH.

$ allenap-flock-experiment
Experiment with flock(2) on a file named `LOCK` in the current directory. Try:

  o – to open the lock file (do this first)
  c – to close the lock file
  s – to acquire a shared lock
  S – to acquire a shared lock without blocking
  x - to acquire an exclusive lock
  X - to acquire an exclusive lock without blocking
  u - to unlock
  U - to unlock without blocking
  q - to quit.
  h or ? - for help.

Opening lock file...
-> Opened lock file
Obtaining shared lock...
-> Obtained shared lock.
Obtaining exclusive lock...
-> Obtained exclusive lock.
Unlocking...
-> Unlocked.
Bye.

Above, I pressed the keys o, s, x, u, then q to exit.

That's all there is to it. However, it's more interesting to run it in a second terminal at the same time, or even in a third. This should give you an idea of how acquiring and releasing locks works in practice. Things to try:

• Compare the normal and the non-blocking modes.
• Run allenap-flock-experiment in three terminals at the same time, and acquire a shared lock in each. Then see what happens as you try to acquire an exclusive lock in each.
• Acquire an exclusive lock in one terminal, then try to acquire an exclusive lock in another terminal. In the first, switch to a shared lock. The documentation hints that the first may lose its hold on the lock altogether, but see what happens on your machine.